Magnetron frequency stabilization apparatus



March 3, 1953 M. A. HERLIN 2,530,533

MAGNETRON FREQUENCY STABILIZATION APPARATUS Filed Oct. 10, 1945 I l i Q l2 l5 l6 l /2 FIG. 2

ROUND POSTS l3\ l5 I i WAVE GUIDE l4 I I1 OUTPUT l 1 2 2 COAXIAL LINE OUTPUT SOURCE OF MAGNETI C FIELD IO ll FIG.|

ATTORNEY Patented Mar. 3, .1953

MAGNETRON FREQUENCY STABILIZATION APPARATUS Melvin A. Herlin, Cambridge, Mass., assignor, by mesne assignments, to the United States of America as represented by the Secretary of War Application October 10, 1945, Serial No. 621,564

8 Claims.

This invention relates in general to magnetrons, and more particularly to means for stabilizing the operating frequency thereof, especially with regard to plural cavity magnetrons.

The conditions which affect the operating frequency of a magnetron include load conditions, temperature of the magnetron elements, and pushing.

The effect of load conditions on magnetron operating frequency may be expressed in terms of the pulling figure. One definition for pulling figure is the value of the maximum change of frequency experienced by a magnetron.

when in its output line a voltage standing wave ratio of 1.5 is moved 360 in phase. Reduction in pulling figure may be obtained by decoupling the output line from the magnetron to a greater extent. This procedure is undesirable in that the circuit efliciency is reduced in this operation and, further, that the electronic efilciency of the magnetron falls off as the load is reduced.

The effect of temperature on magnetron operating frequency is ordinarily given by multiplying the magnetron frequency by the coefficient of thermal expansion of the material of which the resonant members are constructed, the result being, say, the number of megacycles per degree of temperature shift. The severity with which the pulling figure affects the magnetron frequency as compared to the effect of temperature depends in the main on the operating frequency.

Pushing, which is the effect of the parameters within the magnetron on the operating frequency, depends upon the individual magnetron. In generaLthe effect of pushing is quite small with respectto the effects of pulling and temperature change. U

The term stabilization, as used in connection with magnetron operation, refers to improvements in frequency constancy with regard to changes in operating conditions. Any magnetron is stable to some degree because there is associated with it a resonant circuit which stores electrical energy at its resonant frequency. It can be shown that the degree to which a system tends to operate near this frequency is a function of the amount of energy stored in the resonant circuit. Improvements in frequency constancy, therefore, may be looked for from a device which stores additional energy at the desired frequency of operation. With regard to the type of magnetron having its anodestruc "ture comprising a number of cavity resonators,

total energy stored in system energy stored in unaltered magnetron That frequency changes due to varying operating conditions are reduced by this factor (provided the frequency of the stabilizing cavity is not affected by these changes) can be seen. from Equation 2, page 229, in the book by E. A. Guillamin entitled Communication Networks, volume II published by John Wiley and Sons, and the explanation connected therewith.

Among the objects of the present invention, therefore, are:

1. To provide a magnetron oscillator for use at extremely high frequencies;

2. To provide such an output circuit for this magnetron that an appreciable degree of stabilization may be obtained; and

3. To provide such an output circuit embodying a relatively simple structure.

In accordance with the present invention. there is provided a plural cavity magnetron having its output coupled to a rectangular wave guide line.

-Positioned at a maximum point in the standing wave pattern in the line is a conducting post, placed parallel to the electric field, and in the center of the wide dimension of the wave guide. One-half a guide wavelength from this post is positioned another conducting post placed in the same manner. The region between these two posts acts as a stabilizing cavity resonator for the magnetron. The wave guide is also used as the output line.

This invention will best be understood by reference to the drawing, in which Fig. 1 is a functional diagram of a possible application of the present type of magnetron stabilizer, Fig. 2 is a cross-sectional view of the wave guide taken along the line of 2-2 of Fig. l.

. ciated source of magnetic field ll.

. coaxial line [2, which couples at its opposite end to a rectangular wave guide I3 by coupling means 14. As represented on the drawing. the electric aeeoasee field within the guide is parallel to the plane of the paper.

Positioned a distance down the wave guide I3 from the coaxial line to wave guide transition section I4 is a round conducting post I5. This post is held in firm electrical contact with the top and.bottom of the wave .rguide I4, and is placedinsubstantially the center of the wide dimension of the guide, this dimension being perpendicular to the plane of the paper. At substantially one-half a guide wavelength Lg from post I5 there is positioned a second conducting post I6, the placement of this post being similar to that of post I5.

Referring now to a description of the operation of the device, and toFig. -1, it must berealized that the diagram is purely functional, and that there is shown only oneof the many possible applications of this type of magnetron stabilizer. The coaxial line may be coupled to one of thec'avity resonators by the forming of the center conductor thereof into a loop and positioning this loop in such .a manner that an appreciable amount of the oscillating magnetic field links it. If the coaxial line is coupled to the wave guide I3 in the manner shown in Fig. 1, the center conductor may be extended beyond the outer conductor to form a probe I4 which preferably is positioned parallel to the electric field in the guide I 3.

The region between the two posts I5 and It acts as a resonant cavity, the Q of which is dependent upon the diameter of the posts. This may be explained by noting that the larger the diameter of the posts, the more cavity surface there is present for currents to flow over, and thus the greater the losses in the circuit and the less the Q.

One desirable arrangement of the present device is one in which the cavity formed by the two posts has the same effective Q as that of the magnetron itself. By this means a stabilization factor of 2 is obtained, since the stored energy is proportional to the effective Q.

One procedure for determining the desirable positions of the conducting posts is as follows. The magnetron cavities are excited by an outside source of electromagnetic energy, the source being such that the frequency may easily be varied. A standing wave detector such as shown on p. 484 in the book Ultra-High Frequency Techniques, Fifth Printing, by Brainerd et al., may be used in-association with the wave guide I3 for the easy determination of standing wave ratios and position of the maximum and minimum of the waves themselves. The wavelength of the energy exciting the magnetron is varied and the wavelength at which the standing wave ratio goes to a minimum is noted. This is the resonant wavelength of the magnetron operated in this manner. At this resonant wavelength the position of one of the minimums in the standing wave pattern existing in the wave guide is determined. This minimum point represents a point of minimum impedance in the guide. One quarter guide wavelength from this point is a point of maximum impedance. An impedance maximum at the resonant wavelength is a characteristic of a parallel resonant circuit, which the cavity effectively represents, so at this point in the wave guide one of the conducting posts is placed. A corresponding post is placed one-half guide wavelength from this initial post. The region between the two posts stantially at a maximum point in the '4 then acts as a resonant cavity, the Q of which is determined by the diameter of the posts. The external Q of the magnetron cavity may be determined from the graph of standing wave ratio versus exciting wavelength. The post diameter may then be selected so that the Q of the wave .guide cavity is substantially equal to the Q of the magnetron cavity. A stabilization factor of 2 is thus obtained.

In effect, realizing a stabilization factor of 2 means that heavier loading of the magnetron and, hence, greater efiiciency for the magnetron can be secured without sacrificing a low pulling factor for the magnetron. For example, at some frequencies at which plural cavity mag- .netrons operate the magnetron cavity may be so closely coupled to a line that a pulling factor at the magnetron of 20 to 30 megacycles is present. This pulling efiect will be reduced to 10 to 15 megacycles by the addition of this very simple double-post wave guide cavity. In actual practice, the output from one magnetron showed an increase of about 15% with the maintenance of a pulling factor of 10 to 15 megacycles.

While there has been described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall Within the true spirit and scope of the invention.

What is claimed is:

l. A stabilized magnetron oscillator unit comprising a magnetron, rectangular wave guide means having a voltage standing wave pattern in the electromagnetic field therein, means coupling said wave guide to-said magnetronso that said wave guide transmits all the radio frequency energy developed by said magnetron, and means for stabilizing the frequency of the magnetron including two conducting posts havl ing a substantially round cross section positioned symmetrically in said wave guide in the vicinity of said magnetron and substantially parallel to the electric field therein, the distance between the two posts along the wave guide being substantially equal to one-half the wavelength within the wave guide of the radio frequency energy developed by said magnetron, one of said two posts being positioned substantially at a maximum point in the voltage standing wave pattern within said waveguide, whereby said posts act to stabilize the operating frequency of said magnetron.

A stabilized magnetron oscillator unit comprising a magnetron, rectangular wave guide means having a voltage standing wave pattern in the electromagnetic field therein, means coupl ng said wave guide to said magnetron so that said wave guide transmits all the radio frequency energy developed by said magnetron, and means for stabilizing the frequency of the magnetron including two conducting posts positioned in said wave guide in the vicinity of said magnetron and substantially parallel to the electric field therein, the distance between said two posts along the wave guide being substantially equal to one-half the wavelength within the wave guide of the radio frequency energy developed by said magnetron, one of said two posts being positioned sub- I voltage standing wave pattern within said wave guide,

whereby said posts act to stabilize the operatin frequency of said magnetron.

3. A stabilized magnetron oscillator unit comprising a magnetron, wave guide means having a voltage standing wave pattern in the electromagnetic field therein, means coupling said wave guide to said magnetron so that said wave guide transmits all the radio frequency energy developed by said magnetron, and means for stabilizing the frequency of the magnetron including two conducting obstructions positioned in said wave guide in the vicinity of said magnetron, the distance between said obstructions along the wave guide being substantially equal to one-half the wavelength Within the wave guide of the energy developed by said magnetron, one of said two obstructions being positioned substantially at a maximum point in the voltage standing wave pattern within said wave guide, whereby said obstructions act to stabilize the operating frequency of said magnetron.

4. A stabilized magnetron oscillator unit comprising a magnetron, wave guide means coupled to said magnetron for transmitting all the radio frequency energy developed thereby, and two conducting obstructions positioned in said wave guide in the vicinity of said magnetron, the distance between said obstructions along the wave guide being substantially equal to one-half the wavelength within the wave guide of the energy developed by said magnetron, the positioning of said obstructions being such that an effective cavity resonator tightly coupled to the magnetron is formed thereby, said cavity acting as an effec-- tive parallel resonant circuit with respect to said magnetron, whereby said obstructions act to stabilize the operating frequency of said magnetron.

5. A stabilized magnetron oscillator unit comprising a magnetron, wave guide means coupled to said magnetron for transmitting all the radio frequency energy developed by said magnetron, and a plurality of conducting obstructions in said wave guide in the vicinity of said magnetron, the geometry of said obstructions being such that together they form an eifective cavity resonator, the positioning of said obstructions being such that the eifective cavity resonator formed thereby is tightly coupled to the magnetron and acts as an effective parallel resonant circuit with respect to said magnetron, whereby said obstructions act to stabilize the operating frequency of said magnetron.

6. In combination, a rectangular wave guide,

wave guide and substantially parallel to the electric field therein, the distance between the two posts along the wave guide being substantially equal to one-half the wave length within the wave guide of the electromagnetic energy therein, one of said posts being positioned substantially at a maximum point in the standing wave pattern within said wave guide, so that said posts together with the portion of the wave guide therebetween coact as an effective cavity resonator for the electromagnetic energy in the wave guide.

7. A wave guide adapted to transmit electromagnetic energy having two narrow metallic members therein positioned along the median of one wall of said wave guide, the distance between said members along the wave guide being substantially equal to one-half the wave length of the electromagnetic energy in the wave guide, one of said two metallic members being positioned substantially at a maximum voltage point in the standing wave pattern within said wave guide, so, that said members together with the portion of the wave guide therebetween coact as an effective cavity resonator for the electromagnetic energy in said wave guide.

8. A wave guide adapted to transmit electromagnetic energy having a plurality of narrow conducting obstructions therein positioned along the median of one wall of said wave guide, the dimensions of said obstructions and the spacing therebetween being such that said obstructions together with the portion of the wave guide therebetween form an effective cavity resonator for the electromagnetic energy in said wave guide.

MELVIN A. BERLIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,088,749 King Aug. 3, 1937 2,155,508 Schelkunoff Apr. 25, 1939 2,396,044 Fox Mar. 5, 1946' 2,406,402 Ring Aug. 27, 1946 2,432,093 Fox Dec. 9, 1947 2,485,029 Bradley Oct. 18, 1949 2,485,030 Bradley Oct. 1 8, 1949 2,485,031 Bradley Oct. 15, 1949 2,473,448 Rieke June 14, 1949 2,501,052 Herlin Mar. 21, 1950 2,540,488 Mumford Feb. 6, 1951 OTHER REFERENCES Radar System Fundamentals, War Dept. TM 11-467 p. 207. 

